Radar detection and dynamic frequency selection for wireless local area networks

ABSTRACT

A system for detecting and avoiding interference with radar signals in wireless network devices is described. The receiver circuit of the device receives incoming 5 GHz traffic. Such traffic could comprise both WLAN traffic as well as radar signals from radar systems. The incoming packets are treated as an input event, and are screened to be examined as radar pulses. Radar pulses are identified using the length of the detected event. The radar pulses are examined using frequency domain analysis, and the packet train is examined to find gaps between radar pulses. The periodic nature of the packet is determined using frequency domain and time domain analysis to calculate the period of the pulse train. Particular intervals within the pulse train are analyzed using threshold numbers of periodic pulses within the interval and threshold power levels for the pulses. The calculated period information is used to identify the radar source and screen non-radar traffic.

FIELD OF THE INVENTION

[0001] The present invention relates generally to wireless networks, andmore specifically to a system for detecting radar signals using dynamicfrequency selection.

BACKGROUND OF THE INVENTION

[0002] Wireless Local Area Network (WLAN) devices must coexist withradar in the 5 GHz frequency bands. Interference mitigation techniquesare required to enable WLAN devices to share these frequency bands withradar systems. The general requirement is that these devices detectinterference, identify the radar interfering sources, and avoid usingthe frequencies used by the radar. Dynamic Frequency Selection (DFS) isused as a spectrum sharing mechanism by certain standards committeesthat define rules dictating the use of the 5 GHz space. For example, theEuropean Telecommunications Standards Institute (ETSI), which isinvolved in developing standards for Broadband Radio Access Networks(BRAN), requires that transceiver equipment for use in HIPERLAN (HighPerformance Radio Local Area Networks) employ DFS mechanisms to detectinterference from other systems to enable avoidance with co-channeloperations with these other systems, notably radar systems. The goal isto provide a uniform spread of equipment loading across a number ofchannels, such as fourteen channels of 330 MHz each, or 255 MHz each forequipment used only in bands 5470 MHz to 5725 MHz.

[0003] Present proposals from the ETSI BRAN committee provide varioussimple guidelines for radar detection. These include detecting andavoiding radar signals that only appear at a level above a certainpre-defined threshold, such as −62 dBm. In one implementation, detectionis based on a simple algorithm to see whether there are any instances ofsignals above the −62 dBm threshold during a ten second startuplistening period. Another proposed guideline is that detection duringnormal operation should be addressed by periodically suspending allnetwork traffic and listening in startup mode for any instances ofsignals above the −62 dBm threshold level.

[0004] Despite the simple guidelines proposed by the present standardscommittees, the radar and satellite industries increasingly expect 5 GHzWLAN devices to detect radar signals during normal operation. Thus, WLANstations will need to detect radar when they are both transmitting LANpackets, and when they are idle. This will increase the chance ofquickly detecting radar sources that are passing through an area ofoperation of the WLAN device, and thereby reduce interference with suchradar sources.

[0005] However, the present proposed methods of radar detection andavoidance within the 5 GHz space present certain disadvantages,especially in view of increased network traffic in the 5 GHz radiospectrum, and the need for increased bandwidth among WLAN devices. Forthe simple threshold check method proposed by the ETSI BRAN committee, asignificant disadvantage is the possibility of detecting false positivereadings if threshold levels for signal detection are set too low.Another disadvantage includes the inability to effectively distinguishoverlapping cells that may be operating co-channel during a measurementperiod. Moreover, with regard to WLAN equipment operation, therequirement to detect radar signals during normal LAN operation byperiodically suspending network traffic can place a significant burdenon the processing and data transfer capacity of these devices.

[0006] What is needed therefore, is a system that efficiently andaccurately identifies radar in a WLAN device, and allows the device toswitch frequency channels without imposing an undue burden in trafficprocessing throughput. Other objects, features, and advantages of thepresent invention will be apparent from the accompanying drawings andfrom the detailed description that follows below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention is illustrated by way of example and notlimitation in the figures of the accompanying drawings, in which likereferences indicate similar elements, and in which:

[0008]FIG. 1 illustrates a wireless LAN network comprising two WLANAccess Point networks and a potentially interfering radar source;

[0009]FIG. 2 is a general circuit diagram illustrating receiver circuitfor a networked WLAN device that includes a radar detection circuit,according to one embodiment of the present invention;

[0010]FIG. 3 is a general illustration of a typical radar signal thatmay be transmitted by a radar source;

[0011]FIG. 4 is a flowchart that illustrates a method of detecting andavoiding interfering radar signals in a WLAN receiver, according to oneembodiment of the present invention;

[0012]FIGS. 5A, 5B, and 5C illustrate the association of time stampswith periodic events, and a corresponding time line and frequencyspectrograph for an exemplary pulse train signal;

[0013]FIG. 6 is a table of parameters and exemplary values for a processof calculating the periodicity of radar signal patterns, according toone embodiment of the present invention; and

[0014]FIG. 7 illustrates a typical access sequence for a DistributedCoordination Function (DCF) media access mechanism that can be used withembodiments of the present invention.

SUMMARY OF THE PRESENT INVENTION

[0015] A system for detecting and avoiding interference with radarsignals in 5 GHz frequency bands is described. In one embodiment, thereceiver circuit of a wireless LAN (WLAN) device receives incoming 5 GHztraffic. Such traffic could comprise both WLAN traffic as well as radarsignals from radar systems. The incoming packets are treated as an inputevent, and are screened to be examined as radar pulses. Radar pulses areidentified using the length of the detected event. The radar pulses areexamined using frequency domain analysis, and the packet train isexamined to find gaps between radar pulses. The periodic nature of thepacket is determined using frequency domain and time domain analysis tocalculate the period of the pulse train. Particular intervals within thepulse train are analyzed using threshold numbers of periodic pulseswithin the interval and threshold power levels for the pulses. Thecalculated period information is used to identify the radar source andscreen non-radar traffic. In one embodiment, network load reductionschemes are used to provide increased time to measure and analyze theincoming signal pulses. These schemes include the use of beacons toclear traffic for a period of time, increasing time between packets inthe network, and reducing the load after preliminary detection is found.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] A radar detection and avoidance system for wireless networkdevices is described. In the following description, for purposes ofexplanation, numerous specific details are set forth in order to providea thorough understanding of the present invention. It will be evident,however, to one of ordinary skill in the art, that the present inventionmay be practiced without these specific details. In other instances,well-known structures and devices are shown in block diagram form tofacilitate explanation. The description of preferred embodiments is notintended to limit the scope of the claims appended hereto.

[0017] Aspects of the present invention may be implemented within thehardware circuitry and/or software processes of a WLAN or Radio LAN(RLAN) device operating in the 5 GHz space. For purposes of thefollowing discussion, the terms “Wireless LAN” and “Radio LAN” are usedinterchangeably to refer to a network for a device or devices thattransmit in the 5 GHz space. Such a device could be an Access Point(AP), mobile terminal (node), or some other station within a greaterwireless network. The wireless network device is configured to receivenetwork traffic from other WLAN devices. However, it can also receiveunwanted signals from other sources, such as a radar source operating inthe same frequency bands. These signals represent interference, and inthe case of radar signals, the wireless device must take measures toavoid transmitting on the same frequency bands as the interfering radarsources.

[0018]FIG. 1 illustrates the coverage area overlap and interferenceproblems associated with 5 GHz WLAN systems. In system 100, twoindependent networks 103 and 105 are installed near to each other.Within their respective coverage areas, access points (AP) 102 and 104provide access to a fixed backbone network such as an Ethernet LAN or anIEEE 1394 network. Each network 103 and 105 also includes a number ofmobile terminals (MT) wirelessly coupled to their respective networkaccess points. Each mobile terminal can associate and dissociate withaccess points in the radio coverage area. The two radio coverage areas Aand B are shown to overlap, thus illustrating the possibility ofinterference between the WLAN devices (mobile terminals and/or accesspoints) in the coverage areas. The core fixed networks for the accesspoints are in general not the same, and therefore there is nocoordination between the two independent coverage areas. DigitalFrequency Selection (DFS) within each independent wireless network maybe used to control the radio frequency to allow independent WLANs toco-exist in overlapping zones. DFS techniques allow each access point tochoose a frequency with sufficiently low interference; and othermechanisms, such as Transmission Power Control (TPC) reduces the rangeof interference from terminals, increasing spectral efficiency via morefrequent channel re-use within a given geographic area.

[0019] As illustrated in FIG. 1, a radar system 107 comprising a radarsource 106 operating in coverage area C may also overlap one or more ofthe coverage areas operated by an access point. The radar source couldbe a fixed radar source, such as a radar transmitter, or it could be amobile radar source, such as an airplane. The overlap between coveragearea C and coverage area B illustrates potential radar interference withthe WLAN traffic between access point 102 and its respective mobileterminals. In one embodiment of the present invention, access point 102includes a radar detection and avoidance system that enables the WLANsystem 103 to detect the interfering radar signals, possibly identifythe radar source 106 (if its signature or profile is known), and switchto a channel that is free of the radar interference.

[0020] Radar Detection Method

[0021] For system 100 in FIG. 1, access point 102 includes a radardetection system that detects the presence of interfering radar signals.It is assumed that the access point equipment operates in the frequencyranges of 5.15 GHz to 5.35 GHz. This frequency range is generallydivided into ten channels of 20 GHz each. Of these, typically eight areavailable for use by the access point. Upon initialization, for a givenchannel, the access point listens to detect whether any radar signalsare present. If a radar signal is present, the access point WLAN deviceswitches to another channel, until it finds one that is free of radarsignal traffic. This allows the dynamic selection of frequencies withinthe 5 GHz frequency space to avoid interfering with radar sources.

[0022]FIG. 2 is a general circuit diagram illustrating a receivercircuit for a networked WLAN device, such as access point 102, thatincludes a radar detection circuit, according to one embodiment of thepresent invention. Input signals are received by antenna 202 andprocessed through a physical layer comprising amplifier 204 and digitalprocessing circuit 206. The input signals are then processed by MediaAccess Control (MAC) block 208 and further processed by a protocolengine 212. A radar detection process 210 interfaces with the mediaaccess control block 208. This process can be implemented either as asoftware program or module executed by a processor within the wirelessdevice, or it may be implemented as a dedicated hardware circuit coupledto the MAC layer block, or as a combination of software and hardware.The radar detection block 210 executes radar detection algorithms andprocesses that allow the wireless device to detect and avoid interferingradar signals.

[0023]FIG. 3 is a general illustration of a typical radar signal thatmay be transmitted by a radar source, such as radar system 107 inFIG. 1. The radar signal 300 consists of a series of pulses 302,transmitted in a series of bursts, such as first burst 304 and secondburst 306. The bursts are separated by a gap 308. Each radar signalpulse 302 is a high-frequency (approximately 5 GHz) sine wave, and has apulse duration (W) of approximately one microsecond to fivemicroseconds. The pulse period is the time between the start ofconsecutive pulses and is the inverse of the pulse repetition frequency(PRF). The pulse period is typically on the order of one millisecond.The burst length (L) refers to the number of pulses in a burst or thetime duration associated with the burst of pulses. The burst interval(P) is the time from the start of one burst to the start of the nextconsecutive burst, and is on the order of one second to ten seconds.

[0024] In one embodiment of the present invention, the radio receivercircuit 200 of the access point listens for wireless LAN data packets.It is also configured to detect radar signals, such as illustrated inFIG. 3 while it is waiting to receive and respond to normal WLANtraffic. Upon detecting an event, the receiver analyzes the incomingsignal to determine whether or not it is a regular WLAN packet. Varioustypes of unrecognized events can be detected by the receiver. Theseinclude, noise fluctuations, collisions between WLAN stations or hiddennodes, co-channel interference, and other non-LAN wireless traffic, suchas cordless phone transmissions, and the like.

[0025] As can be seen in FIG. 3, radar signals possess a degree ofperiodicity with respect to pulses and bursts. This characteristic isused by the receiver circuit 200 to differentiate noise and other typesof anomalous (non-WLAN traffic) events from radar signals. Althoughnoise may interfere with the WLAN traffic to the same extent as incidentradar signals, noise generally does not need to be avoided to the sameextent as radar signals. Thus, the access point need not be configuredto strictly change channels when encountering noise, as opposed toradar. To properly identify the received non-WLAN signal as radar, theevent is analyzed with respect to periodicity, pulse characteristics,burst characteristics, and other similar parameters in apattern-matching type of process to determine whether the event is aradar signal or not. Different types of radar systems and sourcespossess different pulse and burst characteristics. The system could beconfigured to classify any type of periodic event as a radar signal, orit could be configured to identify, to a certain degree of specificity,the identity of the radar source using look-up tables or profile dataprovided by system operators.

[0026]FIG. 4 is a flowchart that illustrates general process steps in amethod of detecting and avoiding interfering radar signals in a WLANreceiver, according to one embodiment of the present invention. In step402, the receiver screens packets to be examined as radar pulses. Innormal operation, the access point receiver discards all packets thatare not destined for the access point's MAC address and that do notsuccessfully pass a Frame Check Sequence (FCS) error check. If theaccess point is set to “promiscuous mode,” this screening is disabled.When in this promiscuous mode, any event that causes the receiver tosense that a signal is present above a noise floor will be reported to asoftware driver within the radar detection process 210. This noise flooris measured in dBm, and is an adjustable parameter. The report willinclude an estimate of the size of the event and an indication of whenthe event occurred. If the detected event is a valid LAN (e.g., 802.11a)packet with no errors, the packet is processed normally.

[0027] If the MAC circuit 208 determines that the packet has errors,such as physical (PHY) errors (severe signal errors), or CRC (CyclicalRedundancy Check) errors (bad data), it is treated as a potential radarsignal, since the presence of such errors indicates that the event ismost likely not a valid WLAN packet or is a valid packet that iscorrupted by the presence of other signals. Depending upon theconfiguration and desired sensitivity of the access point receiver, theMAC circuit can be configured to allow for further analysis signals withPHY errors only, signals with either PHY or CRC errors, or signals withPHY errors only above a certain threshold. This threshold could be setto a particular value, such as −62 dBm, or it could be adapted dependingon the noise in a given environment. Although CRC errors typicallyindicate that a valid WLAN packet simply has bit errors, a CRC errormight also indicate that the LAN packet was corrupted by an interferingradar signal, and therefore provides evidence of radar interference.Thus, in certain circumstances, received signals containing CRC errorsmay also be passed by the MAC layer for further analysis.

[0028] The reported packets are sent from the MAC layer 208 to the radardetection process 210. The radar detection process then sorts throughall of the reported events to identify radar signals, as shown in step404 of FIG. 4. In general, the process entails first sorting valid WLANpackets. The length of the packets can be calculated based on the lengthand rate at which they are received. The received power is indicated,and the number of packets received can be determined. This informationis then used to build a histogram describing the percentage of time thechannel is occupied with valid LAN traffic at each power level. If suchinformation is gathered on several channels, the histograms are usefulfor choosing the channel with the least interference from other WLANsystems.

[0029] Once the valid WLAN packets are sorted, any non-LAN packet eventsare sorted (binned) into rough magnitude ranges. A periodigram is thencalculated for each of the magnitude bins, indicating any periodicitiesthat are present in the events. In general, radar signals show two basicperiodic natures. First, a given pulse burst (e.g., 304) has shortpulses at a particular frequency (period). Second, the pulse burststhemselves have a quiet interval (e.g., gap 308) between them. Thereforethe pulse bursts themselves have a periodic nature.

[0030] The output of the periodigram is examined for a number ofcharacteristics. First it is determined whether the periods correspondto a known radar system as provided in radar look-up tables or throughradar test signals. If the event does not match a known radar system,the periodicity may indicate that the source is an unknown radar system.Although WLAN traffic is generally not periodic, certain circumstancesmay arise when such traffic does exhibit some degree of periodicity. Forexample, traffic in a neighboring WLAN base station may be present anadjacent channel, in which case the receiver may trigger on thesesignals but does not get valid LAN packets. Or, weak signals received inthe same channel may not be received as valid LAN packets, but may stillregister as weak events.

[0031] As an event is received, a timer circuit within the receiver 200time stamps any pulses or spikes. In general, WLAN traffic typicallyincludes a time code or time indicator. Radar signals, however, do notinclude such time information. To provide time information, a counterwithin the receiver assigns a time stamp to each detected event.

[0032] The receiver circuit 200 includes a Fast Fourier Transform (FFT)engine that analyzes the incoming signal to derive phase and magnitudeinformation of the signal within fine frequency ranges. In oneembodiment, the non-LAN packet events are processed with an FFT, theoutput of which is binned into 52 bins of 300 kHz each. Examining thecontents of these bins (a spectrogram of the received signal) helps toidentify and distinguish among different types of radar, such as CW(continuous wave) tone radar, and chirping radar in which the pulses areswept across a range of frequencies. In the case of radar, the power istypically concentrated in one particular bin, that is, a specificfrequency. This is in contrast to normal LAN traffic in which the powerlevel among all of the bins is roughly equal.

[0033] The packet is also analyzed to determine whether there are anyspikes within the packet above a certain threshold, such as −62 dBm. Aspike might indicate a radar signal that interferes with a LAN packetthereby causing CRC errors in the packet. Such a radar signature isoften hard to detect and characterize. In this case, the amplitude andduration (pulse width) of the spike is analyzed to determine whether ornot the interference is due to a radar pulse. Spikes within packets arealso time-stamped so that the spike can be treated as a new or separateevent.

[0034] Once a particular event is determined to likely be a radar signalthrough an analysis of the length and magnitude of the event, the periodof the signal is determined. This is illustrated as step 406 in FIG. 4.In one embodiment of the present invention, the periodic nature of theevent is determined by performing Fast Fourier Transform analysis of thetime stamped events. A time scale is defined and populated with theoccurrence of events at particular times. An example of this process isillustrated in FIGS. 5A and 5B, in which five events are assigned timestates denoted T1 through T5, as shown in Table 500. FIG. 5B illustratesa time line that is used to code the time relationship of the events. Abinary value “1” is assigned to the time associated with each timestamp, indicating the occurrence of an event at that particular time. Avalue of “0” is assigned to all other time increments on the time line.An FFT calculation is then performed on the entire time line todetermine the frequency at which the events occur. The resultingspectrograph will reveal the periodic nature of the time-stamped events.An example of a spectrograph that may correspond to the frequencytimeline of FIG. 5B is illustrated in FIG. 5C.

[0035] In an alternative embodiment of the present invention, the periodof the events is determined using Discrete Fourier Transform (DFT)analysis. In the DFT analysis, only the time intervals that have anevent associated with it are analyzed. These are compared with specificfrequency bins to determine whether or not there is any correlation. Thecorrelation of each of the events is then added together to determinethe period of the events. The DFT method is typically more efficientwhen the time between pulses is long (e.g., on the order ofmilliseconds) compared with the resolution of the time scale (e.g., onthe order of microseconds). Like the FFT method, the DFT methodconstitutes a frequency domain technique for analyzing the periodicnature of the event.

[0036] A further alternative embodiment for determining the period ofthe event is a time domain analysis of the event. In this method, theinterval between the first pulse and the second pulse is assumed to bethe period. The subsequent pulses are then analyzed to determine whetherthey occur at the same period. If the subsequent pulses do not occur atthe same period, the interval between next pair of pulses is thenassumed to be the period against which other pulse intervals arecompared. If the new assumed period is not successfully matched, theprocess is repeated using other intervals as the base period, until amatch among the pulse intervals is found. When implemented in a digitalprocessing system, the time domain analysis can prove to becomputationally more efficient than the FFT and DFT methods. This isespecially true when the pulses are spaced far apart in time relative toa fine resolution frequency domain.

[0037] In some circumstances, the resolution of the counter thattime-stamps the events may not correspond to, or be as fine as theresolution of the frequency bins. For example, a timer that only has aresolution of one microsecond may not provide enough resolution tocapture the exact beginning of an event. The resolution of the timerthus effects the shape of the spectrograph obtained through bothfrequency and time domain analysis techniques. Therefore, a mechanism isprovided to allow for an error margin associated with the time stamp.For the FFT and DFT frequency domain techniques, the error marginassociated with the time stamp can cause the frequency distributioncurve to be shorter and wider due to small frequency shifts caused byimprecise time measurements. For these methods, the error margin isdetermined relative to the resolution of the frequency bins so that therelative error taken into account in determining the period of theevents. Any resulting diminution in the peak of the frequency graph isthen compensated for in accordance with the relative margin. Thus, ifthe error margin due to the time stamp resolution is known, a certaindegree distortion (i.e., widening) of the frequency curve can beexpected and accounted for.

[0038] In the time domain analysis technique, the method of accountingfor the error margin associated with the time stamp is the associationof a window with the assumed period derived by the event intervals.Thus, if the first and second intervals occur at a particular period, aplus or minus margin is associated with this period to account for theerror margin of the time stamp. The interval between the nextconsecutive events is then compared to this period with the margin forerror.

[0039] The next step in the flowchart of FIG. 4 is to analyze theperiodicity of the pulses over a single time interval, step 408. Thetime interval is set to a value roughly the same as the period overwhich the pulses occur, that is, the burst length, L. This interval isthen examined to determine whether a particular number of periodicpulses occurs within the interval. The search for periodicity can beperformed as described previously by comparing the time between pulses.If periodic pulses are found, then it is assumed that a radar signal ispresent on that frequency channel. For example, the occurrence of fiveperiodic pulses within an interval may signify a radar signal withinthat interval of time. A threshold value for the power level of thepulses is also defined. If frequency domain techniques are used todetermine if the pulses are periodic, an FFT or DFT process is performedfor each time interval as it is shifted along the time scale.

[0040] In step 410, the signal is analyzed over multiple intervals. Thisstep compensates for any shortcomings associated with analyzing thesignal over a single interval. For example, a particular interval mayonly include three pulses, while the threshold number of pulses is five.In this case, a radar signal would not be identified for that interval.However the next consecutive interval may include another three pulses.In this case, the number of pulses for both intervals is six, which isabove the threshold number to qualify as a likely radar signal. Step 410essentially provides a historical analysis of multiple intervals andcompensates for the fact that the start and end points of the intervalsare somewhat randomly defined relative to the pulse events. This stepalso compensates for occurrences in which radar pulses are missed, thuscausing gaps in the pulse train. Radar pulses might be missed in severaldifferent types of circumstances. For example, a station generallycannot detect radar pulses when it is transmitting, or when it isconcurrently receiving a strong signal. When the signal analysis isperformed over multiple intervals, the threshold number of pulses perinterval is reduced. If the threshold is set too low, the possibility offalsely detecting radar increases, due to the possible periodic natureof random noise or other types of incident signals. Similarly the powerthreshold for the pulses is also reduced as the number of intervalsincreases.

[0041] As can be seen in FIG. 3, there can be more than one degree ofperiodicity within a radar signal. There is a period associated with thepulses, as well as a period associated with the bursts of pulses, thusresulting in a “nesting” of periodicity. In one embodiment of thepresent invention, the single and multiple interval analysis performedin steps 408 and 410 are checked to see if there is any recurrence ofpulse detection over a certain period of time. For example if aparticular interval yields five pulses at a particular time, T, and thenanother five pulses at a time T+100 milliseconds, then it is likely thata pulse train similar to that illustrated in FIG. 3 is detected.

[0042] Once the period of the detected signal is calculated, the periodcan be matched to known radar signals to verify that the signal isindeed a radar signal. This is illustrated as step 412 of FIG. 4. In anembodiment of the present invention, it is assumed that any periodicevent is a radar signal. This reduces the need to match the period toknown radar to verify that the signal is a radar signal. Once the periodis known, this information can also be used to eliminate periods thatcorrespond to natural periods in LAN data traffic, step 414. AlthoughWLAN traffic, such as 802.11 traffic is technically random, andtherefore non-periodic, certain natural periods can arise depending uponthe type of traffic that is transmitted. If the period of the detectedevent matches the known natural period of the LAN traffic, the signalcan be classified as LAN traffic as opposed to radar traffic that shouldbe strictly avoided.

[0043] A second method of avoiding radar signals that can constitute asecondary check for the period determination method described above isto count the number of PHY error events that occur in a given period oftime. In this method, a threshold number of PHY errors is set for agiven time interval. As a signal is received by the receiver, the numberof PHY errors is counted. If the number of detected PHY errors occurs ina particular period of time, it is assumed that the signal is a radarsignal.

[0044] Known Period Detection

[0045] As described above, one method of determining the period of theradar signal employs a time domain analysis in which the interval oftime between two consecutive pulses is iteratively compared amongdifferent pairs of pulses, as shown in step 406 of FIG. 4. The followingdiscussion provides a detailed description of the implementation ofproviding known period detection according to a preferred embodiment ofthe present invention. It should be noted that although specific codeand programming structures may be used to describe the method, these areprovided for purposes of illustration, and other program structures maybe used. For this embodiment, the radar detection process 210 detectsthe presence of radar signals using well-known patterns of pulserepetition. The duration of time between two consecutive pulses isdetermined and the process searches for a minimum number of error eventsthat fall on the known time boundaries.

[0046] In one embodiment, the radar detection process 210 includes analgorithm that uses hard-coded data structures to hold information aboutthe radar patterns (PPS) that it is looking for. Event information isstored in an array as time deltas between consecutive events. A searchis performed every time a new event is added to the array (once enoughdata points are collected). A time delta is considered to match theperiod of a particular radar pattern if the product of frequency, i.e.,PPS (pulses per second) and time delta (in microseconds) is a multipleof 1,000,000. This value is called “tfValue” in the routine providedbelow. The resulting value is compared against its rounded value withdesired percentage margin. This principle is used at several places inthe algorithm. The advantage to this approach is that if pulses in thepulse sequence were missed, the periods between the remaining pulseswill appear as an integer multiple of tfValue. Therefore, it is easy tocheck if the pulses are within the margin and therefore periodic, evenif some pulses are missing from the pulse stream.

[0047] The receiver circuit 200 indicates a PHY error when interferencesuch as a radar pulse hits the receiver. It may result in indicating azero length frame to the host if the interference activates thereceiver. Any interference that hits the receiver while it is receivingthe payload of the frame causes a CRC error. The hardware generates aninterrupt to the host on every received frame that has an error. Theinterrupt handler, “ar5hwcEndIn( )” reads the current timestamp from thehardware registers. This may be in microseconds. Alternatively, thetimestamp value may be placed in the receive descriptor status field bythe physical layer of the receiver, in which case the time stamp isgenerally more accurate. Reading the timestamp in the interrupt handleris acceptable as long as the interrupt latency is approximately the samefor all events. Hence, the timestamp is captured at the highestinterrupt priority.

[0048] The algorithm uses timestamps only for PHY errors and not for CRCerrors. Since a CRC error can be caused by interference anywhere in themiddle of a frame (stretching across a few milliseconds), theirtimestamps generally do not closely represent timing of theinterference. PHY error timestamps are entered in the event array atsystem task priority by the “ProcessRxInterrup( )” process.

[0049] The following software routine provides an example of the radardetection algorithm utilizing time domain frequency analysis. Thisalgorithm is based on hierarchically searching subsets of possible radarfrequencies. For example, in the first pass the possible frequencies arestepped in large steps. For example, all frequencies from 300 to 4000 Hzare checked by stepping the frequency by 100 Hz each time. Because thesteps are large, the allowable margin in the tfValue must be large todetect any pulse repetition frequencies that lie between the coarsesteps. Once a suspected regular pulse period is found, for examplebetween 500 and 600 Hz in this example, that range is then more finelyscanned, with a tighter margin on the tfValue. In this case the secondscan might run from 500 to 600 Hz in 10 Hz steps with the tfValue marginreduced by a factor of 10 by the first sweep. This hierarchical searchcan be continued until the desired tfValue margin is achieved. Onceachieved, the events are known to be periodic within a given accuracy,and the frequency of the events is determined with the desired accuracy.for each pps in PPS_ARRAY for each event in event array discard firstfew events that are outside burst time delta tfValue = timedelta * ppsstore it in pulse1 array for each event in pulse1 array if differencebet rounded and original tfValue is within wideMargin store it in pulsearray if pulse has minimum no. of pulses // Now use narrow margin forsmall variations of pps around the current pps value. for each finerincrement within pps repeat the above loops, but for narrow margin. ifpulse has minimum no. of pulses radar signal is detected

[0050] The program loops include filters to remove undesired data pointsin order to reduce the false alarm probability. If the tfValue of adatapoint is zero, it is less than 50% of the period that is beinglooked for, and such points are discarded. If tfValue is not within themargin but it is off by more than 30%, it counts as a mismatch. If thenumber of mismatches is equal to or exceeds the number of matches, it isa susceptible event sample. If tfValue is not 1 million, but is insteada multiple of a million (2, 3, 4 million, etc.), it is likely that thereceiver dropped some interference events or this pattern may match aPPS that is one-half or one-third of the current PPS. If more than halfof data points exhibit this characteristic, the sample is discarded.

[0051]FIG. 6 is a table of parameters and exemplary values for a processof calculating the periodicity of radar signal patterns, according toone embodiment of the present invention, and is used to represent theradar signal patterns. The frequency, totalNumPulses and silentPeriodparameters are taken from the definition of that signal likerepresentative radar. The rest of the parameters can be used to tune thesensitivity of the algorithm, false alarm probability and variation offrequencies. Most of these parameters can impact the performance of thesystem. The code excerpt below illustrates a programming structure thatutilizes the data of table 600, according to one embodiment of thepresent invention. // Information about radar patterns typedef structppsArray { A_INT32 arraySize; A_INT32 count; struct ppsInfo { A_INT32freq;        // value of pps A_INT32 totalNumPulses; // burst sizeA_INT32 silentPeriod;// scan rate in seconds A_INT32minPulsesToDetect;//min pulses for ID A_INT32 stepFrom;// finer searchfrom this pps A_INT32 stepIncr;// with this increment A_INT32 stepTo; //to this pps A_INT32 wideMargin; //wide margin%*1000 for first searchA_INT32 narrowMargin; //narrow margin%*1000 for finer search }info[PPS_ARRAY_SIZE]; } PPS_ARRAY;

[0052] The initRadarDetect( ) initializes the event array and computesthe maximum burst time of all the PPS's. It is used to age out thoseevents from the array that fall outside this time delta. It also enablespromiscuous mode and zero length frame indication in the hardware ofcircuit 200.

[0053] The following routine initiates processing of PHY errors atruntime. It is called in the background while other tasks are running.The access point initialization code goes through the basic channelinitialization and waits for one minute, and keeps checking the flag ifradar is detected. ar5hwcEndInt ( )looks for PHY error interrupt. Readstimestamp and stores it locally. ProcessRxInterrupt ( ) recordRadarEvent( ) to enter the time delta in event array processRadarEvents ( ) toperform required processing radarEventAge ( ) to filter out eventsoutside max burst duration checkRadarPattern ( ) knownPeriod ( ) - Lookfor known radar pattern apReboot () - Reboot the AP on radar detection(optional)

[0054] The apReboot could be substituted with marking a channel asunusable due to the presence of radar, and then randomly choosinganother channel to begin sampling to determine if radar is present.Detecting radar during network initialization may be easier becausethere is no network traffic flowing at that time that could obscureradar pulses. Therefore the thresholds and listening periods can beadjusted accordingly.

[0055] Reduction of Network Load

[0056] During high traffic conditions, in which the access point istransmitting and receiving a large percentage of the time, the radardetection system may not be able to adequately detect and identify allpossible radar interference events. In one embodiment of the presentinvention, the radar detection process is used in conjunction withprocesses that reduce the network load of the access point by increasingthe amount of free air time. The increased free air time increases thelikelihood that radar pulses will be detected by the receiver. This isillustrated as step 416 in FIG. 4. The network load reduction techniquesare closely related to the Media Access Control mechanisms provided inthe 802.11 standard.

[0057] One media access mechanism provided for in the 802.11 standard isthe Distributed Coordination Function (DCF), which is commonly known as“listen before talk.” For DCF, the random access is slotted, with arandom back-off time selected within a contention window following abusy medium condition. In addition, all directed traffic uses immediatepositive acknowledgment (ACK frame) where re-transmission is scheduledby the sender if no ACK is received. FIG. 7 illustrates a typical accesssequence for a DCF media access mechanism that can be used withembodiments of the present invention. A Short Inter-Frame Spacing (SIFS)is used between a packet and its acknowledgement (ACK). After theacknowledgement packet, other receivers wait for the period of aDistributed Inter-Frame Space (DIFS), as well as a random portion of thecontention window (CW). For the DCF method, one way to reduce thenetwork load is to increase the time between packets by increasing thecontention window. This increases the time that nodes must wait beforetransmitting and thus reduces the load on the network. Similarly, SIFand DIF periods can be changed to create more free network time. Anenhanced DCF method in which nodes are given different contentionwindows based on the priority of traffic can also be used with the radardetection system.

[0058] Another type of media access mechanism provided for in the 802.11standard is the Point Coordination Function (PCF). The PCF accessmechanism is based on polling by the access point. In this mode, theaccess point controls which nodes transmit and when they transmit. Theaccess point first issues a PCF beacon, which announces the beginning ofa polling period and informs all other nodes that they must wait untilthey are polled before transmitting. After that, the access point cancommence polling of the nodes. Polls can be combined with data payloads,as well as with acknowledgments of previous packets. Once polled, thenodes can respond with combined data and acknowledgements. Thus, the PCFmechanism is a permission-based mechanism in which the access pointexplicitly allows nodes to transmit using the PCF beacon. The gapsbetween packets in this mode are generally SIFS or PIFS, although theaccess point can always leave the medium idle for any length of time itchooses.

[0059] During a PCF polling period, if the access point does not poll astation, there should be no network traffic, thus providing a period ofquiet time on the network. In one embodiment, the access point isconfigured to send a PCF beacon on a periodic basis, such as once every100 milliseconds. During this time, in which the network traffic shouldbe ceased, the access point can perform a radar detection process. Forthis embodiment, the access point first sends a PCF beacon, thenexecutes a radar detection cycle. At the end of the radar detectioncycle, the access point can then proceed with normal PCF polling. Thisallows network traffic to flow at a regular pace, albeit perhaps at areduced throughput rate, rather than being completely stopped for aperiod of time. For the PCF method, the SIF and DIF periods can bechanged to create more free network time.

[0060] In an alternative embodiment, the access point can be configuredto hold traffic at the access point prior to transmission, or to commandthe nodes to hold traffic prior to transmission. For this embodiment,the access point transmits a command instructing the nodes to nottransmit again for a certain period of time after a previoustransmission. This serves to slow traffic from the nodes at a ratecorresponding to the hold period. One method of implementing thismechanism is for the access point to hold its own acknowledgment and/ordata packets. This causes the transmitting nodes to use a largercontention window. In addition, higher level protocol engines to assumethat there is heavy traffic on the network and slow the trafficaccordingly. The access point could achieve a similar result by jammingthe network with traffic to create collisions. This forces thecontention window to be increased, since it appears that there are toomany nodes for the number of available slots.

[0061] In a further alternative of the present invention, the networkload is reduced by the access point after a preliminary detection ofradar signals is made. For this embodiment, the pulse number thresholdis set at a low value, for example three pulses to trigger the detectionprocess. This allows for the detection of almost all possible radarsignals, as well as the possibility of false positive readings. Upon thedetection of an event, the network load is reduced temporarily using oneof the above described methods, such as increasing the use of the PCFbeacon to clear traffic from the network. During the time of ceased orreduced traffic, the threshold is set to a higher pulse number value,for example eight pulses, and the radar detection process is thenre-executed. With the higher threshold, the possibility of falsepositive readings is decreased. For the preliminary detection period ofthis embodiment, the method of detecting radar signals could be a simplecheck of all pulses above a particular power level, as describedpreviously, as opposed to a full profile analysis using the length ofthe event and an analysis of the periodicity of the event. Once thepreliminary detection is performed at the lower threshold number, thesecond detection process for the higher number of pulses is thenperformed using a determination of the periodicity.

[0062] In one embodiment of the present invention, the radar detectionprocess is executed only at the access point. If the access pointdetects the presence of a radar signal, it will change channels. When onthe new channel, it will listen to see if there are radar signals onthat channel. If not, it begins sending beacons on that new channel.Otherwise it switches channels again and repeats until it finds achannel with no radar signals. The nodes (mobile terminals) meanwhilewill have lost the access point and will eventually search to associate.They will either find another access point with which they canassociate, or will eventually find the access point with which they wereoriginally associated. Although this channel hop is not graceful, it isexpected to be a relatively rare occurrence. Most of the radar sourcesare fixed so they only need to be found and avoided once, typicallyduring the power-on sequence for the access point. It should be notedthat, in general, all nodes are constrained to passive scanning foraccess points. Active scanning, in which the nodes launch probe requestpackets before hearing an access point would not be allowed. Thisprevents a station from interfering with radar systems while trying tolocate an access point. Passive scanning is fully supported by the802.11 protocol, although it is generally considered slower to associatewith this approach.

[0063] In an alternative embodiment, the access point could delegate theradar detection process to one of the nodes. For this embodiment, theaccess point sends a command to a particular node to perform a radardetection process and report the results back to the access point sothat the access point can switch channels to a free channel. In thiscase, the node could use any of the previously described radar detectionmechanisms to find the presence of radar and report that to the accesspoint.

[0064] Uniform Spread of the Loading

[0065] Embodiments of the present invention relating to radar detectionand avoidance also work in conjunction with methods to provide a uniformspread of the loading of the equipment across a minimum number ofchannels in the 5 MHz space. For example, under the 802.11 standard, DFSequipment is required to spread the load across a minimum of 14 channels(or 330 MHz), or 255 MHz in the case of equipment used only in the band5470 MHz to 5725 MHz to facilitate sharing with satellite services.

[0066] In one embodiment, the access point circuit of FIG. 2 operatesfrom 5150-5350 MHz. Therefore, the spread will be across 200 MHz and not330 MHz. However, since the intent of spreading is to minimize the powerspectral density in any given bandwidth, the system compensates by usingreduced power to make up for having less bandwidth to spread over. Thecommensurate power reduction would be approximately 2.2 dB.

[0067] Additionally, an access point that profiles all channels wouldneed to follow this algorithm for selecting a channel to use. First, anychannels in which radar signals are suspected would not be used in anycircumstances. Second, the access point would avoid any channels thatare already heavily loaded with WLAN traffic. Finally, among theremaining channels that have neither radar nor WLAN traffic, the accesspoint would choose randomly. This prioritization and method for finalselection insures the most uniform spreading of the traffic whileminimizing potential interference to radar systems.

[0068] By actively searching for and identifying radar signals, theaccess point provides the best possible radar avoidance. Detection canbe further improved over time if the periodicities of more radar systemsare published. However, even radar systems that are unknown can bedetected as long as they have a periodic nature. The radar-detectionalgorithm and process described herein allows radar detection to beperformed periodically during normal network operation, and possiblyeven as live traffic is being handled. Trading power for spreadingbandwidth is reasonable under these circumstances because the resultingpower spectral density will be the same. Moreover, it allows theconstruction of products that operate only in the lower band. Suchproducts would be directly compatible between present U.S. and Europeanstandards, since the 5150-5350 kHz band has been allocated in bothdomains with similar power limits.

[0069] Although embodiments of the present invention have occasionallybeen discussed in the context of particular 5 GHz WLAN standards, suchas the U.S. 802.11 standard, it should be understood that alternateembodiments can be used in systems that conform to other standards, suchas the European HIPERLAN (High Performance Radio Local Area Network)standard. Some of these standards include their own propositions andrequirements for radar detection and avoidance.

[0070] For example, present HIPERLAN2 radar detection capabilitiespresently being proposed by the ETSI BRAN standards committee aregenerally not as robust as those described herein. These methods arebased solely on measuring if power above a certain threshold was everreceived. This method has a number of drawbacks. First, the thresholdmust be set quite high in order to limit the chance of false positivesin radar detection; second, even with high thresholds the chances offalse positives is significant considering overlapping cells that may beoperating co-channel during the measurement period; and third, such abrute force method cannot be used while live traffic is being serviced.Therefore network designers must either sacrifice throughput or spend asmall fraction of the time searching for radar signals. Since some ofthe radar signals have long periods, this tradeoff may be hard tomanage.

[0071] The analysis of the periodicity of the received event can be usedto extend the radar detection capability of such HIPERLAN systems. Inaddition, this method can be used in conjunction with certain featuresunique to HIPERLAN systems. For example, HIPERLAN2 provides thefacilities for an access point to command a station to measure adifferent channel and report the result. The implication of this featureis that if for some reason the access point cannot hear the radar, butthe mobile station can, the channel will still be rejected. Althoughsuch a situation might be rare since the radar signals will likely becoming from a much farther distance than the distance between stationsin a WLAN cell, it does provide a method for an access point to delegatethe radar detection and avoidance mechanism to other nodes.

[0072] In addition, the radar detection system can be extended to allowfor the generation of management packets from the access point thatcould be interpreted as commands to nodes to measure the traffic onparticular channels. The stations would then report the results back tothe access point. Similarly, management packets could be defined for theresult reports, including indications of radar, 802.11a traffic, andother WLAN traffic.

[0073] The system could also be extended to allow nodes and accesspoints to generate histograms that show the percentage of time on thechannel that various types of traffic were present in relation to thesignal power level. This provides a measure of time occupancy versussignal strength. The generation of this type of histogram relies uponthe measurement of duration of non-LAN traffic events. Such durationinformation can also be derived for LAN traffic events from the rate andlength information for each packet. A process then generates a histogramfor non-LAN traffic based on the number of events detected by the chip.

[0074] New management packets and new elements in beacons can bespecified to announce an upcoming channel change. This allows thechannel switch to be much quicker, without breaking associations.Additionally, all nodes will know when to switch, and to which channelthey should switch.

[0075] In the foregoing, a system has been described for detecting andavoiding interfering radar signal in WLAN devices operating in the 5 GHzfrequency space. Although the present invention has been described withreference to specific exemplary embodiments, it will be evident thatvarious modifications and changes may be made to these embodimentswithout departing from the broader spirit and scope of the invention asset forth in the claims. Accordingly, the specification and drawings areto be regarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. A method of detecting radar signals in a wirelessnetwork device comprising the steps of: receiving a plurality of signalpulses as detected events; eliminating any detected events thatcorrespond to network traffic; determining whether any non-eliminatedevents of the individual detected events correspond to radar signals byexamining at least one of: a pulse repetition frequency, a pulse period,or a number of pulses in a pre-defined time period.
 2. The method ofclaim 1 wherein only events exhibiting physical errors beyond apre-defined tolerance are examined.
 3. The method of claim 1 whereinsignal pulses that arrive during reception of network traffic areidentified by a sudden increase in received signal strength, relative toa threshold power level, and are identified as individual detectedevents.
 4. The method of claim 1 wherein only events with a signalstrength above a pre-defined threshold signal strength level areexamined.
 5. The method of claim 1 wherein only events that have a pulsewidth duration shorter than a pre-defined pulse width threshold areexamined.
 6. The method of claim 1 further comprising the step of timestamping the events upon reception to produce a time stamp for eachevent of the detected events.
 7. The method of claim 1 wherein at leastone of the pulse period, pulse repetition frequency, number of pulses inthe pre-defined time period is matched to corresponding properties ofknown radar sources.
 8. The method of claim 1 wherein if the pulserepetition frequency of a series of events corresponds to a commonlyoccurring frequency of network traffic, the series is ignored, and theevents of the series of events are eliminated from examination.
 9. Themethod of claim 1 further comprising the step of performing a frequencydomain analysis on a waveform that created the event.
 10. The method ofclaim 9 wherein the frequency domain analysis comprises a Fast FourierTransform analysis.
 11. The method of claim 1 wherein the step ofdetermining the pulse period or the pulse repetition frequency of theremaining events comprises performing a frequency domain analysis on aseries of the remaining events.
 12. The method of claim 11 wherein thefrequency domain analysis comprises Fast Fourier Transform analysis. 13.The method of claim 12 wherein the frequency domain analysis comprisesDiscrete Fourier Transform analysis.
 14. The method of claim 6 whereinthe step of determining a period of the event comprises performing atime domain analysis of the event.
 15. The method of claim 14 furthercomprising the steps of: determining a first time interval between afirst pulse and second pulse of the event; assuming that the first timeinterval represents the period of the event; and comparing the firsttime interval with a second time interval between the second pulse and athird pulse of the event to determine whether the first and second timeintervals match to within an error factor associated with the time stampfor each detected event.
 16. The method of claim 15 further comprisingthe steps of: assuming a pulse repetition frequency to create an assumedfrequency; multiplying a time interval between the pulses by the assumedfrequency to create a result; and determining if the result is within apre-defined margin of being an integer value.
 17. The method of claim 16in which an initial assumed frequency is obtained by inverting the timeinterval between the pulses.
 18. The method of claim 16 in which aplurality of assumed frequencies is created by assuming different pulserepetition frequencies.
 19. The method of claim 17 further comprisingthe steps of: initially selecting widely spaced frequencies and widemargins; determining when a positive result is found; and subsequentlyselecting finely spaced frequencies and margins to confirm the presenceof a radar signal.
 20. The method of claim 1 further comprising the stepof reducing network traffic for the network device.
 21. The method ofclaim 20 wherein the step of reducing network traffic comprises the stepof using a beacon signal to suspend traffic over the network for aperiod of time.
 22. The method of claim 20 wherein the step of reducingnetwork traffic comprises the step of increasing a time between packetsin the network.
 23. The method of claim 20 wherein the step of reducingnetwork traffic comprises increasing a contention window comprising timeslots for carrying network traffic.
 24. The method of claim 1 whereinthe wireless network device comprises an access point station coupled toa 5 GHz radio network, and wherein the network utilizes one of 802.11network protocol and HIPERLAN protocol.
 25. A device coupled to one ormore remote terminals over a wireless network, comprising: a receivercircuit for receiving network data traffic as an input signal from theone or more remote terminals; a media access control circuit coupled tothe receiver circuit, and configured to detect an error contained in thenetwork data traffic received by the receiving circuit, and to classifythe error based on a pre-defined error type; and a radar detectioncircuit coupled to the media access control circuit and configured toreceive the input signal if the error contained in the input signal isof a first error type, the radar detection circuit comprising, a firstprocess receiving a plurality of signal pulses as detected events, asecond process eliminating any detected events that correspond tonetwork traffic, and a third process determining whether anynon-eliminated events of the individual detected events correspond toradar signals by examining at least one of: a pulse repetitionfrequency, a pulse period, or a number of pulses in a pre-defined timeperiod.
 26. The device of claim 25 wherein only events exhibitingphysical errors beyond a pre-defined tolerance are examined.
 27. Thedevice of claim 25 wherein pulses that arrive during reception ofnetwork traffic are identified by a sudden increase in the receivedsignal strength, and are identified as individual detected events. 28.The device of claim 25 wherein only events with a signal strength abovea threshold power level are examined.
 29. The device of claim 25 whereinonly events that have a pulse width duration shorter than a pre-definedpulse width threshold are examined.
 30. The device of claim 25 whereinthe events are time stamped.
 31. The device of claim 25 wherein at leastone of the pulse period, the pulse repetition frequency, and the numberof pulses in the pre-defined time period is matched to correspondingproperties of known radar sources.
 32. The device of claim 25 wherein ifthe pulse repetition frequency of a series of events corresponds to acommonly occurring frequency of network traffic, the series of events isignored, and the events of the series of events are eliminated fromexamination.
 33. The device of claim 25 wherein the error contained inthe input signal comprises a physical error affecting an expectedwaveform characteristic of the input signal.
 34. The device of claim 33wherein the physical error causes waveform distortion exceeding apre-defined tolerance associated with a signal level for the networkdata traffic.
 35. The device of claim 34 wherein the error contained inthe input signal further comprises a bit-error associated with a digitalcontent of the received network data traffic.
 36. The device of claim 25wherein each pulse is considered an event, and the events appear as afirst event burst followed by one or more subsequent event bursts, thedevice further comprising: a time stamp circuit assigning a time stampto each event that is above a pre-defined power threshold; and an eventanalysis circuit defining a threshold number of events within a timeinterval, above which the events are considered to be radar events. 37.The device of claim 36 wherein the examination of events is performed bya Fourier Transform process for frequency domain analysis on the eventsover the time interval to determine a period of the events.
 38. Thedevice of claim 36 wherein the period of the event bursts is determinedby a Fourier Transform process for performing frequency domain analysison the event bursts as detected by the analysis circuit to determine apulse burst period.
 39. The device of claim 36 wherein the period of theevent bursts is determined by a time domain analysis process performedon the event bursts as detected by the event analysis circuit todetermine a pulse burst period.
 40. The device of claim 39 wherein thetime domain analysis process: determines a first time interval between afirst pulse burst and second pulse burst; assumes that the first timeinterval represents the period of the event; and compares the first timeinterval with a second time interval between the second pulse burst anda third pulse burst of the event to determine whether the first andsecond time intervals match to within an error factor associated withthe time stamp.
 41. The device of claim 39 further comprising: a fourthprocess assuming a pulse repetition frequency to create an assumedfrequency; a fifth process multiplying a time interval between thepulses by the assumed frequency to create a result; and a sixth processdetermining if the result is within a pre-defined margin of being aninteger value.
 42. The device of claim 41 in which a plurality ofassumed frequencies is created by assuming different pulse repetitionfrequencies.
 43. The device of claim 42 further comprising: a seventhprocess for initially selecting widely spaced frequencies and widemargins are used; an eighth process determining when a positive resultis found; and a ninth process for subsequently selecting finely spacedfrequencies and narrow margins to confirm the presence of a radarsignal.
 44. The device of claim 25 further comprising a circuit formatching the period of the events to known radar sources.
 45. The deviceof claim 25 further comprising an optimization circuit coupled to theradar detection circuit, the optimization circuit operable to reducenetwork traffic received by the receiver circuit.
 46. The device ofclaim 45 wherein the optimization circuit transmits a beacon signal overthe wireless network to the one or more remote terminals to suspendtraffic over the wireless network for a period of time.
 47. The deviceof claim 45 wherein the optimization circuit increases a contentionwindow comprising time slots for carrying network traffic.
 48. Thedevice of claim 25 wherein the wireless network comprises a 5 GHz radionetwork, and wherein the device and the one or more remote terminalsutilize one of 802.11 network protocol and HIPERLAN protocol forcommunication.
 49. A method of identifying radar signals in a networkdevice coupled to one or more remote terminals over a wireless network,comprising the steps of: receiving an input signal comprising aplurality of pulses as detected events; determining whether the detectedevents correspond to expected network traffic; assigning a time stamp toeach event of the detected events that is above a pre-defined powerthreshold; determining a first time interval between a first event andsecond event; assuming that the first time interval represents theperiod of the events; and comparing the first time interval with asecond time interval between the second event and a third event todetermine whether the first and second time intervals match to within anerror factor associated with the time stamp.
 50. The method of claim 49further comprising the step of defining a period of the events tocorrespond to a time interval between successive pulses that matches thefirst or the second time intervals.
 51. The method of claim 50 furthercomprising the steps of: defining periodic time intervals; defining athreshold number of pulses within each time interval; and defining athreshold power level for the pulses within the time interval.
 52. Themethod of claim 51 further comprising the step of determining whetherthe number of pulses above the threshold power level within a timeinterval meets or exceeds the threshold number of pulses.
 53. The methodof claim 52 further comprising the step of determining whether thenumber of pulses above the threshold power level within two or more timeintervals meets or exceeds a threshold number of pulses, if the numberof pulses within one interval does not meet the threshold number ofpulses for a single interval.
 54. The method of claim 50 wherein thedetected event represents an input signal exhibiting physical errorsbeyond a pre-defined tolerance
 55. The method of claim 49 furthercomprising the step of matching the period of the event to known radarsources.
 56. The method of claim 49 further comprising the step ofswitching an input channel of the network device to a differentfrequency band to avoid interference with the input signal.
 57. Themethod of claim 49 further comprising the step of reducing networktraffic for the network device using one of: a beacon signal transmittedto one or more remote terminals coupled to the network device causingthe one or more terminals to suspend transmission to the network device,and a jamming signal operable to lengthen a contention period of timefor prioritizing network traffic from the one or more remote terminals.58. The method of claim 49 wherein the network device comprises anaccess point station coupled to a 5 GHz radio network, and wherein thenetwork utilizes one of: 802.11 network protocol and HIPERLAN networkprotocol.